RFR: 8315247: GenShen: Condition calls to post-write barrier code generation by a flag

[Kelvin Nilsen]

On Mon, 28 Aug 2023 21:51:31 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> This approach has parity with the other barrier flags. `shenandoahGenerationalMode.cpp` asserts that the flag is on. 
>> 
>> It shouldn't be strictly necessary with the default being `false`, but we may want to have `shenandoahPassiveMode.cpp` explicitly turn this flag off as it does the other barrier flags.
>
> If we make this visible on the command line, we will want to make sure it is consistent with mode->is_generational().  Right?

(or you could say, if they turn off ShenandoahCardBarrier, that would mean we have no remembered set and always scan all of old-gen.  but that would be a lot more work for us, and of no value, I think...)

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/313#discussion_r1307996693